I think both Sam and Dean had reasons to object to bringing Jack back, especially since they didn't know about the whole Empty trying to get him. Now, if they had known that, it would have been a slightly different kettle of fish. I can hand wave that Sam just loves Jack so much that he doesn't care about the consequences. He wants him back and that's that. I can also hand wave that the writers are going for Dean loves Jack as much if not more than Sam so he's cool with the whole resurrection thing. Basically, if Dean doesn't approve, it doesn't happen, so he had to approve. I could not care less about what Cas thinks, so I refuse to hand wave that at all.